1. Field of the Invention
The invention relates to a method for stabilizing the current gain of NPN-transistors.
2. Description of the Prior Art
The degradation of the current gain B, i.e. the ratio of collector current to base current of bipolar transistors is a known problem in semiconductor technology. From IEEE Trans. Electron Devices, Vol. ED 18, No. 8, Aug. 1971, Pages 570 to 573, it is known that a drop of the current gain can be caused not only by mobile ions in the oxide or avalanching the emitter-base diode, but also by reverse temperature stress, called in the following HTRB (High-Temperature-Reverse-Bias). In a considerable percentage of all NPN transistors, HTRB stress results in a linear drop of the current gain, i.e. a drop which is constant over a major range of the collector current. This degradation of the current gain is also called B-drift. This B-drift can be healed by heating under high-current breakthrough load or by annealing without applied reverse voltage, i.e. the original current gain prior to the HTRB is obtained again.
FIG. 1 shows a cross-section through a NPN-planar-transistor to which the process according to the invention is applied. The transistor consists of a n.sup.+ -conducting substrate 5, thereupon an n-conducting collector zone 4, a therein arranged p-conducting base-zone 3, and an also therein arranged n-conducting emitter-zone 2. The emitter zone 2 is contacted by the emitter contact 7. The base-zone 3 is contacted by the base contact 8. The parts of the semiconductor surface 6 which are not covered by the metal contacts are covered by an insulating layer 9. Emitter and base contacts are so constructed that they lie on tope of parts of the insulating layer 9. The dash-lines 10 and 11 indicate the borders of a space-charge zone at the base-collector junction (transition). The width of the space-charge zone depends on potential between collector and base, U.sub.CB. At U.sub.CB =40 V, a width of the space-charge zone results as shown by the lines 10 and 11 in the drawing shown in the scale of the drawing. The dash-lines 12 and 13 indicate the width of the space charge zone at the pn-junction between emitter and base. The width of this space charge zone depends on the potential between emitter and base, which in the case of the lines 12, 13 shown in the drawing is 4 volts.
The contact arrangement of the collector is not shown in the drawing. It can either be made on the backside of the substrate, or n.sup.+ -doped substrate connector-layers can be arranged in the semiconductor chip, so that they reach the semiconductor surface 6, and make the contacting of the collector possible on the surface 6. A forward current stress of the emitter-base diode of transistors "drifted" in this manner leads to a further change in the current gain, the so-called B-elevator effect. The current gain change caused by this effect is non-linear, the transistors show an unstable family of output characteristics on a curve-tracer. Characteristic is a current-gain degradation at small collector currents and a current-gain increase for large collector currents. The B-elevator effect can be initiated not only by HTRB stress, but can also occur, for instance, without electrical stress, as investigations of our own have shown.